I have realised something about the FTC’s behaviour that might make what I want easy.
I’ve just turned on the DHW from timed off and a hot water cycle started even though the temperature was only a couple of degrees under target (set to come on if drops by 10 degrees).
So… I should be able to leave it on with it monitoring the temperature at the top of the tank (so just before we are about to run out).
But I then also set up the schedule to turn hot water off for a bit before the off peak time and then back on.
I think this should allow us to always run hot water during the night but it would also trigger during the day if we are going to run out.
On days were we don’t use as much or there is solar boost then the additional run shouldn’t happen.
I was worried that using the top tank temperature would mean heating of the water would stop too early to heat all of the tank but it appears with our coil base tank the water is agitated enough by thermal currents that this isn’t an issue.
But the stratification in the tank is really good so the bottom temp can be down into the teens when the top of the tank has just dropped a few degrees.
This should let us set an excess solar threshold where under this the immersion will be used as normal but above it the Heat Pump will be triggered to heat the hot water. Need to get this threshold right so it is enough to completely power the heat pump. Were sun is intermittent the heat pump will be allowed to complete a sensible length cycle but this could end up using non solar electricity. However if the threshold is high enough there is a good chance that our battery will be well charge as it is a sunny day and cover any drops in solar rather than using grid. The battery will be set up to take priority over the Eddi for excess solar.
If it doesn’t work out I can always just disable it and use the Eddi and immersion as normal.
I was away for a week recently with the ASHP in holiday mode. When it came off that, due to two days of rain etc. the ASHP DHW tank (also heated by the Eddi during PV excess) was below the lower threshold so it was heated back up to 55C by the ASHP.
That took 26 minutes with a COP of 1.24 so yes, more efficient than direct heating.
One of the reasons I use the Eddi is that both it and the EV charger have dynamic load management which I use to avoid blowing the house fuse overnight when everything is trying to operate during the Go period. If the ASHP isn’t already on (despite overnight temperature setback), it could throw another 30+ A into the mix.
Mm, it would appear our temperature probe pockets are too low and too high.
I’m heating our tank during night low rate electricity but on some days this isn’t enough to get us through the day (250l tank!) so I need to sometimes do another run before the evening. This will also depend on amount of solar diversion once we have that installed.
However if we use the bottom pocket I think the hot water would come on with every shower but in the top pocket the temperature doesn’t drop significantly until it’s too late to heat again.
I’m going to stick with it for a bit and continue to adjust the drop that triggers the HP but assuming this doesn’t work what other option is there?
Is there some way I could measure the actual volume of hot water used since the last run? This would seem like the most accurate way to work out if we are going to run out or not.
How hot are you heating the tank up to? How many kWh are you putting in each night?
Maybe you can use both pockets? Bottom one for heating up the cylinder, top one providing extra info about what’s in the tank. Maybe by analysing both you can decide when to add more heat?
You would need a flow meter on the outlet from (or inlet into) the cylinder to measure exactly how much hot water has been used. Probably best asking how in a new thread.
A cheaper, less invasive, less accurate option might to strap a thermal or acoustic sensor to the pipe to detect when hot water is flowing.
I have been heating to 48 degrees, although I have increased this to 50 while I try to figure our the best thresholds to use.
It might be that eventually the solution is to just have 2 scheduled DHW cycles and lower the temp to 45 degrees so they are as efficient as possible. Probably with something to stop the 2nd one if the solar diverter has heated up the bottom of the tank during the day.
The Ecodan only has one probe that I access the value for through pymelcloud. However it is possible to connect 2 probes to the FTC for larger tanks but I don’t know if it can monitor both or it is just that you can select between it using one or the other from the controller.
Anyone have the 2 probe setup? I think it might be how the larger Ecodan cylinders are set up so anyone with one of those able to comment on how this works?
I think to do what you suggest I would need to add to my external monitoring setup. I currently have a EmonHP and a second Pi that is connected to our solar invertor. What would the simplest way of setting up 2 temperature probes ideally using the hardware I already have?
Another thought I have had about the positioning of a probe to decide when to boost the tank is whether it could be attached to where the connection to the coil comes out. This is near the middle of the tank so might give a more useful reading. As the probe isn’t in a pocket within the tank it wont be a very accurate temperature but it might give a reading I can set a good threshold on.
Over the last few days I’ve been experimenting with attaching the temperature probe to the capped off pipe that comes out of the cylinder for a secondary hot water return.
This is a bit further down the cylinder than the top sensor pocket so has been causing the Ecodan to kick in earlier to heat water when needed so we don’t run out.
A possible down side (although I need more runs to verify) is that as the probe isn’t more ‘in’ the cylinder I think there is a temperature reading lag when heating up. I think this might be meaning the flow temperature is higher and I would assume the actual tank temperature is overshooting a bit.
Maybe I can reduce the temperature to counter that a bit but then that might start effecting the trigger for starting the tank warm up.
Our Electric Meters are hopefully being installed tomorrow so I will be able to see the effect on consumption soon.